Investigation of stomach content has traditionally been an important field of activity
in fisheries biology, but it is one in which there are great difficulties in correlating
the results with the research made in the other fields. Investigations of the food of the
fish cannot be considered in isolation but have to be discussed in relation to the whole
marine environment, of which the fish constitute single elements. Therefore, a brief survey
of the most important processes in aquatic ecology must be made, with particular reference
to feeding.

Living organisms interact with each other and with their non-living (abiotic) environment
in many ways; no organism exists independently of its environment. It is the study of
these interrelationships which is called ecology. It is possible to study the ecology of
one species in relation to its environment or a whole group of species and their interactions
with both each other and their physical surroundings. Thus, ecology is concerned
not only with the biological disciplines but also the physical and chemical sciences.

Any area of nature in which materials are being exchanged between living organisms and
their abiotic environment forms an ecological system or ecosystem. This concept is useful
as it stresses the interdependence of the components involved. Although it is hardly possible
to demarcate any area in nature that is not influenced by neighbouring areas, one may,
nevertheless, consider a pond, a lake or even part of a forest as an ecosystem.

To understand the dynamics of such a self-sufficient ecosystem as a functional unit,
its component parts must be looked at in some detail first.

Consumption of the organic material by heterotrophic (other methods of nourishment)
consumers.

Decomposition of the organic material to inorganic nutrients, suitable for the producers,
by saprophytic micro-consumers, chiefly bacteria and fungi.

The main producers are the chlorophyll-possessing green plants; synthesizing bacteria
usually play only a very minor role.

The consumers comprise all the other living components present. They include the
herbivores (plant-eaters), which feed directly on the producers, and the carnivores
(predators), which feed on the herbivores or other carnivores. They also include parasites,
scavengers, (carrion-eaters) and saprophytes. Although it is convenient to list the
decomposers, consisting of bacteria and fungi, as a separate entity because of their specific
role and their indispensability they are of course also consumers.

The inorganic nutrients comprise a large number of elements present in the form of dissolved
salts. The most important are nitrogen and phosphorus followed by potassium, calcium,
sulphur and magnesium. Some elements are needed in extremely minute amounts and are therefore
referred to as micro-nutrients.

The production of organic substances (food) by photosynthesis is a process involving
transformation of light energy into potential chemical energy. The transfer of this food
energy from the producers through a series of consumers is called a food chain, each organism
through which it is passed being a link in the chain.

For sake of simplicity three different food chains may be recognized:

The carnivore chain, where the energy is passed from smaller to larger organisms.

The parasite chain, where the energy is passed from larger to smaller organisms.

The saprophyte chain, where the energy is passed from dead organic matter to micro-organism
in most cases.

In reality food may be passed through parts of all three chains before it is finally
decomposed into inorganic nutrients by the bacteria and fungi found at the end of every food
chain. In other words, the species population within a community or ecosystem form many
food chains which interconnect, anastomose or cross each other in a complex pattern, which is
usually referred to as the food web.

Organisms which belong to the same link of the food chain as counted from the producer
level are said to belong to the same trophic level. Thus the plants constitute the first
trophic level, the herbivores the second, and the carnivores feeding on herbivores the third
trophic level. Secondary carnivores feeding on third level carnivores belong to the fourth
trophic level and so forth. However, there is a very definite limit to the number of possible
links in a food chain, and consequently also to the number of trophic levels in any
ecosystem. The reason for this is that only about 10 percent of the available energy is
assimilated in passing from one trophic level to the next. At the top of the food chain
there are usually only one or two major predators. The number of species in each trophic
layer increases with approach to the first layer, giving rise to what is called a pyramid of
numbers. For the major predators introduction of small amounts of pollutants into the first
trophic layer can have fatal consequences because it is eventually concentrated in them.

The laws of thermodynamics state that energy cannot be created or destroyed, but also
that it cannot be transformed from one type to another without partial dispersion into heat
energy. This means that the transformation of light energy into potential chemical energy
in the form of organic compounds in the plants cannot be 100 percent efficient. Only a very
small portion of the light energy absorbed by green plants that is transformed into food
energy (gross production) because most of it is dispersed as heat. Furthermore, some of the
synthesized gross production is used by the plants in their own respiratory processes, leaving
a still smaller amount of potential energy (the net production) available for transfer to the
next trophic level.

The loss of energy is generally referred to as the respiratory loss because the organisms
utilize the food energy by oxidizing it. Because of the respiratory losses the food chains
cannot be very long and the number of trophic levels in natural communities is therefore
seldom more than four or five and often only three. It also means that the total amount of
food available decreases with increasing trophic level. For this reason, the largest
animals are found feeding on either plants or other animals which are in a low trophic level
as, for example, whales on krill and elephants on plants.

Among animals the gross production corresponds to the food assimilated, which means
food ingested and absorbed by the intestine. The net production is here equal to food
assimilated minus respiration.

While most of the energy lost within an ecosystem is due to the respiratory processes,
there are other losses which affect the individual organisms. Some of the potential food is
not ingested, but is either decomposed directly or is stored or is exported out of the system
or community. Another source of loss is that not all of the food ingested is actually assimilated;
some passes through the alimentary canal and is lost as faeces.

As stated earlier, as the organisms die they are attacked by the decomposers, which derive
their energy from them by reducing their organic contents to inorganic nutrients. As
also indicated earlier, these nutrients can then be used by the producers anew with the result
that the materials involved are continuously circulating in the system. However, the
energy flow is strictly passed along a “one-way street”. To keep an ecosystem going light
energy must be continually supplied.

As we have seen, true production of organic matter takes place only in the chlorophyll-possessing
plants and certain synthetic bacteria, and this has been referred to as the
primary production. However, copepods and euphausids, for example, are often referred to as
meat producers or “key industry” animals because they convert plant material into protein
that can be assimilated by the animals which eat them but which themselves could not exist
on plant material. In reality, of course, they only assimilate and store energy derived
from the primary producers. To avoid confusion it would be better to call them secondary
producers, a term which of course fits animals at higher trophic levels just as well because
they too - although indirectly - utilize the primary production of the plants.

From a practical point of view it is often desirable to find out how big is the
secondary production of certain animals in a given area, say a fishing bank, or even more
important, whether a known production can be increased. Production estimates must be based
on such factors as standing crop (biomass), rate of removal of materials and rate of growth,
including growth of young born or hatched during the census period. The turnover rate is
also of interest when short-lived species are involved as is practically always the case
within ecosystems in the sea. The biological production must be expressed per time unit.
A large standing crop is by no means synonymous with a large production rate. To take an
easily visualized example, a pasture grazed by cows may have a very small standing crop of
grass because the production is being eaten as it is being produced, but it may nevertheless
have a higher production rate than a neighbouring ungrazed pasture with a very large standing
crop.

Quantitative relations between the various trophic levels can be calculated provided the
production rates are known for each level concerned. Relationships of this nature within
trophic levels are also of considerable interest. Expressed as percentage ratios the results
of such calculations are often referred to as ecological efficiencies because they are
concerned with the efficiency of energy transfer at different points along the food chain.
Thus they are important to our understanding of the dynamics in ecosystems. Moreover, most
of the efficiency ratios are meaningful with regard to single species populations as well as
to whole trophic levels.

Unfortunately much confusion exists in the terminology used by various authors, and it
is not always clear to which efficiency ratio an author has really wished to refer. Odum
(1959) has made an attempt to define the various ratios, based on his energy flow diagram,
and this is certainly a good method of illustrating the complexities involved (Fig. 6.3).

Among fishery biologists the most common way of describing the efficiency is by the
conversion factor, i.e. the ratio of the weight of the food consumed by the fish and the
growth in weight. Some authors express this conversion factor as the nutritional
coefficient. From Fig. 6.3 it is seen that the conversion factor is the reciprocal value of
what Odum calls the ecological growth efficiency. The value of the conversion factor is
traditionally defined as 10, but in fact it can be both much higher and much lower, e.g. in
trout farms in northern Europe the conversion factor is about 5.

However, it is important to stress that none of the ecological efficiencies are constant
for any species population or for a whole trophic level. They are dependent on a number of
abiotic factors such as temperature and salinity, as well as biotic factors such as type,
abundance and distribution of available food, and the age of the consumers; for example,
larvae and young ones of all species investigated have much lower conversion factors than
older animals.

Almost all the work that has been done upon the food intake of fishes has been qualitative,
rather than quantitative. That is, workers have described the occurrence of food
found in the digestive tract, usually in the stomach only. This tells what the fish has
eaten and approximately in what proportions but it does not describe how much of each food
species is eaten. The reaon for the lack of quantitative work is that it is very time-consuming.

As soon as possible after the fish has been caught all the digestive tract is dissected
out, and if the investigation cannot be finished at once, possibly because of lack of
laboratory facilities, the material must be preserved in 4 percent buffered formalin.

Later in the laboratory the different sections of the digestive tract are opened and
the individual groups of organisms are sorted out foridentification. This is most easily
done with a binocular-dissecting microscope. To illustrate the methods in use and to describe
their advantages and disadvantages a series of results will be considered based on
the examination of the stomach contents of two Lates niloticus (Table 1).

The occurrence of each food item is recorded and expressed as the percentage occurrence
of all food organisms. The method is very quick and easy but it underestimates the importance
of the larger species. In the example, 40 percent of all the food occurrences were Odonata
nymphs. The stomach contents of these two L. niloticus could have consisted almost entirely
of nymphs, each with a very small Tilapia galilaea but the results give no indication whether
this might be a true interpretation of the data.

The numbers of each food item are recorded and the results expressed as a percentage of
the total number of food items present. This method overestimates the importance of small
food organisms, in this case the Odonata nymphs. It is not satisfactory with plant food,
except with microscopic examination of algae. In the example given the plant material could
be a small piece of grass or a large piece.

The method is similar to the frequency of occurrence method except that the results are
expressed in terms of the number of fish in which the food item occurred expressed as a percentage
of the total number of fish. The method gives an indication of what are the main
food species but does not indicate their relative importance as sources of energy.

The entire volume of the stomach contents is measured. The contents are then sorted
into different types of food and the volume of each determined. Results are usually reported
as percentage of the total volume.

This method is more time-consuming than any of the previous methods but it describes
accurately the relative importance of each food species and the total volume (∑) indicate the
quantity of each item being eaten.

This method is identical to the previous method except that the volume of each food item
is first expressed as a percentage volume of the total stomach contents from which it was removed
and then an average taken for all the fish sampled. The advantage of this method over
the previous one is that the final results will be more representative of the feeding habits
of a group of fish if one or two individuals in that group have been feeding very heavily on
one particular food item. The disadvantage is that the information on the actual volume of
the stomach contents is lost.

Total weights of all food items are determined. This method is very similar to the
volumetric method, over which it has little advantage unless the sample is dried so that only
the dry weight is determined. Results are usually presented as percentages of the total. If
wet-weights are determined the food items must be placed on a blotter for a moment to remove
excess moisture. This problem of removing excess moisture makes the method somewhat subjective
and therefore dry-weight is better, although much more time-consuming.

Points are given to each item. The number of points depends upon both whether the
organism is very common in the stomach contents (highest number of points) or rare (lowest
number) and upon its size (more points for large than small size). The method may also be
modified to take stomach fullness into account. It is rapid, easy and requires no special
apparatus; with experience the method can be very accurate. In the example shown (Table 6.1)
the points allocated give a result similar to both the volumetric and gravimetric methods.

Further detail about these methods is given by Rounsefell and Everhart (1953).

All these methods describe, with a greater or lesser degree of precision, the amount of
food which the two L. niloticus, which were used as an example, had in their stomachs; the
best being the volumetric and gravimetric methods. The sum of the actual volumes and of the
weights also indicates the degree of fullness of the stomach, which enables feeding cycles
to be described. The points system is also good if used objectively and has the added
advantages of ease and quickness. It can also be adapted to take into account fullness of
the stomach.

However, none of these methods describes the quantity of each food item being eaten.
In the example used, T. galilaea may take a long time to digest and be identifiable long
after Odonata nymphs cease to be. During the time it takes to digest one T. galilaea 20–30
Odonata nymphs might have been eaten and digested but at any one time only 4–6 might be
recognizable in the stomach. On the other hand, some food items might have been regurgitate
upon capture. While the qualitative methods allow the construction of food chains they do
not describe the energy flow through those food chains nor allow the importance of say, predation
by one commercially-valuable species on another equally valuable species, to be determined.
If answers to these problems were known it might in some instances be considered
advisable to reduce the abundance of a predator species to allow an increase in abundance of
its prey, especially if the prey were at a lower trophic (more efficient) level. These
questions can be answered by qualitative studies only.

Quantitative feeding studies take considerable time and are therefore the work of the
specialist. Only a brief outline of the methods used will be given. Firstly the unit in
which energy flow is to be measured is chosen, calories and nitrogen being the two commonest.
Secondly, feeding experiments are conducted in aquaria with the different food materials which
the species under study eats in order to determine how efficient it is in converting the food
item into fish flesh. For calories, this means determining the calorific value of the selected
food, feeding the fish a known weight of this food, collecting its faeces and determining
the calorific value of these. In order to translate field observations on the weights and
calorific values of food found in stomachs digestion rates must be established by feeding fish
and then killing them at known intervals. This may need to be done for more than one temperature.
From this information is built up a picture of the daily food intake of fish of various
sizes of the same species, how much of the energy from this food it utilizes to maintain
itself and to grow. One such study by Daan (in press) shows that large cod in the southern
North Sea probably have a big influence on the size of year-classes by eating small O-group
cod and that they also eat large quantities of herring, and soles. Boddeke (1971) has also
shown that recent large year-classes of cod have reduced the abundance of brown shrimp
(Crangon crangon) off the Dutch coast. Daan has suggested that the total production of commercial
species from the southern North Sea might be increased by reducing the abundance of
cod by heavy fishing upon the stock. Solution of such predator-prey relationships might
alter the manner in which some fish assessments were made.

Boddeke R., 1971, The influence of the strong 1969 and 1970 year classes of cod on the stock of
brown shrimp along the Netherlands coast in 1970 and 1971. ICES C.M. Shellfish
and Benthos Committee. Paper K32:3 p. (mimeo)